In a typical cellular network, also referred to as a communications system or a communications network, one or more User Equipments (UE) communicates via a Radio Access Network (RAN) to a Core Network (CN).
A user equipment is a mobile terminal by which a subscriber may access services offered by an operator's core network and services outside the operator's network to which the operator's radio access network and core network provide access. The user equipment may be for example a communication device such as a mobile telephone, a cellular telephone, a smart phone, a tablet computer or a laptop with wireless capability. The user equipment may be portable, pocket-storable, hand-held, computer-comprised or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as for example another user equipment or a server.
The user equipment is enabled and configured to communicate wirelessly in the communications network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, comprised within the communications network.
The communications network covers a geographical area which is divided into cell areas, and may therefor also be referred to as a cellular network. Each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. evolved Node B (eNB), eNodeB, NodeB, B node or Base Transceiver Station (BTS), depending on the technology and terminology used. The base station communicates over the air interface operating on radio frequencies with the user equipment(s) within range of the base station.
Modulation is used in communications networks and relates to the way information is superimposed on a radio carrier. A radio carrier needs to be modulated so that it may convey information from one device to another.
One type of modulation is Quadrature Amplitude Modulation (QAM). QAM conveys two analog message signals or two digital bit streams, by changing, i.e. modulating, the amplitudes of two radio carrier waves, using an Amplitude-Shift Keying (ASK) digital modulation scheme or an Amplitude Modulation (AM) analog modulation scheme. The two radio carrier waves are shifted in phase by 90° and modulated, and are therefore called quadrature carriers or quadrature components. The modulated resultant output comprises both amplitude and phase variations.
There are different forms of QAM, such as e.g. 16 QAM, 32 QAM, 64 QAM, 128 QAM, and 256 QAM. The numbers 16, 32, 64 etc. refer to the number of points in a constellation diagram. A constellation diagram is a representation of a signal modulated by a digital modulation scheme such as QAM. It displays the signal as a two-dimensional scatter diagram in the complex plane at symbol sampling instants. It may also be seen as a representation of the possible symbols that may be selected by a given modulation scheme as points in the complex plane.
In a communications network, the user equipment and the base station communicate by means of several channels. The channels are used to separate different types of data. The data channels may be grouped into three categories: a physical channel, a transport channel and a logical channel. Each category of data channel may be uplink or downlink, where uplink is in the direction from the user equipment to the base station and where downlink is in the direction from the base station to the user equipment. The logical and transport channels define which data is transported, while the physical channels define with which physical characteristic the data is transported. In more detail, the transport channel provides transportation of data to Medium Access Control (MAC) and higher layers. A transport block comprises data bits and is an entity exchanged between the MAC and the physical layer via the transport channel. The number of bits in a transport block is referred to as transport block size. A transport block may be segmented into code blocks. In each Transmission Time Interval (TTI), the MAC delivers a given number of transport blocks for each of the transport channels multiplexed together according to a Transport Format Combination (TFC) to the physical layer. The physical layer executes a set of processes to map the transport blocks onto the available physical resources (i.e. carrier frequency, code sequence and radio frame). Segmentation of the bit sequence from transport block concatenation is performed if X_i>Z, where X_i is the number of bits input to the segmentation and Z is a maximum code block size. The number of code blocks is C_i=X_i/Z. The code blocks after segmentation are of the same size.
A transport format is a format offered by the physical layer to the MAC, and vice versa, for the delivery of a transport block during a TTI on a transport channel.
Hybrid Automatic Repeat reQuest (HARQ) is a technique used to detect and correct errors. If the channel quality is sufficiently good relative to the transport format selected, the incurred transmission errors are correctable and the receiver is able to decode the transport block correctly. In that case, an ACKnowledgement (ACK) is sent by the user equipment and received by the base station. If the channel quality is poor, relative to the transport format selected, not all transmission errors may be corrected, the receiver will request a retransmission of the transport block, i.e. no ACK is received. The received data from the retransmission is combined with the data from the first (or previous) transmissions stored in the HARQ buffer to improve the successful decoding probability. Some aspects of the first transmission and of the retransmissions are determined by a Redundancy Version (RV) parameter controlled by the base station. Support signalling for the HARQ function involves transmitting for example the redundancy version parameter and information about the transport block size. In chapter 4.6.2.1 (“Redundancy and constellation coding”) in the Third Generation Partnership Project Technical Specification (3GPP TS) 25.212 the version parameter parameter is described for Wideband Code Division Multiple Access (WCDMA)/High-Speed Downlink Packet Access (HSPA). HARQ is used in standards such as e.g. WCDMA, HSDPA and Long Term Evolution (LTE).
As wireless standards evolved, higher order modulations were introduced to increase the amount of data transmitted per time unit and bandwidth, such as e.g. 16QAM and 64QAM. For efficient processing, the large amount of transmit data is divided into several smaller transmit data segments, after which, each of them has been individually forward error correction encoded. In case of WCDMA this corresponds to the segmentation of a transport block into code blocks, and Turbo encoding of the code blocks. While the maximum transport block size grew with evolving standards, the maximum code block sizes remained the same, and the maximum code block size is 5114 bits in the example of WCDMA.
A soft bit value (aka. a soft bit, a soft value, a Log-Likelihood Ratio (LLR) value) is a real number which indicates the likelihood that the given bit was sent as 1 or 0. A large negative value, for example, −100, may imply a high likelihood for the bit is 0 and +100 may imply a high likelihood for the bit to be 1. The bits carried by the QAM symbols have different reliabilities. The soft bit values, describing the transmitted bits with different reliability, are partitioned between code blocks in a transport block. In 16QAM and 64QAM modulations not all de-mapped soft bits carry the same quality and reliability in a decoding process, due to the way the bits are mapped on to a constellation diagram. For example in 64QAM, soft bits index 0 and 1 define the I/Q half-planes and are the most reliable bits. Soft bits index of 2 and 3 toggling two times per dimension (using two decision boundaries) have medium reliability and soft bits index 4 and 5 toggling four times per dimension are the least reliable. The soft bit distribution in code blocks is the result of a bit recollection stage described in a 3GPP technical specification. The soft bits may be stored in a dedicated soft bit buffer or in any suitable computer readable memory.